Semiconductor devices are fabricated by depositing layers of semiconductor films and metal interconnects. The layers may be deposited using a multistation sequential deposition system. In a multistation sequential deposition system, multiple silicon wafers are processed sequentially, receiving a portion of the total deposition thickness of a given layer at separate stations in the system. As wafers move to successive stations, a new wafer is introduced to the process chamber while a completed wafer is removed. When a new wafer is introduced at the first station, the wafer is heated to process temperature at the first station before processing begins.
When a new wafer is placed on the pedestal at the first station, the difference between the temperature of the wafer and the temperature of the heated pedestal can cause the wafer to deform. For example, 300 mm silicon wafers have been observed to bow when placed upon a heated pedestal in a multistation deposition system in a vacuum based process chamber. FIGS. 1A-1C show a wafer warping on a heated pedestal in a deposition system. FIG. 1A shows the system at time 0, just after wafer 12 has been placed on pedestal 10. Wafer 12 has a center 12a and an edge, 12b. At time 0, both center 12a and edge 12b of wafer 12 contact pedestal 10. FIG. 1B shows the system at time t1, after wafer 12 has been heating for time t1. Wafer 12 has bowed so that center 12a still contacts pedestal 10, but edge 12b has lifted off pedestal 10 by a distance 14. Distance 14 may be about 0.08 inches between the edge of wafer 12 and pedestal 10. FIG. 1C shows the system at time t2, after wafer 12 has been heating for time t2. The wafer has now heated up enough that the wafer has relaxed from the bowed state shown in FIG. 1B. Once a wafer has bowed, it may take 80-120 seconds for the wafer to heat up enough from the heated pedestal to relax.
One cause of the warping is nonuniform heating of the wafer. FIG. 2A shows a graph 20 with two curves, one (21) showing the temperature of the center of the wafer and one (22) showing the temperature of the edge of the wafer at a given time after the wafer is placed on heated pedestal 10 of FIG. 1. The wafer in FIG. 2A is heated in oxygen. FIG. 2A shows that the center of the wafer heats much more quickly than the edge of the wafer. FIG. 2B shows the temperature difference between the center and the edge of the wafer at a given time after the wafer is placed on the pedestal (curve 26). The temperature difference peaks at about 130 degrees Celsius about ten seconds after the wafer is placed on the pedestal. This temperature difference causes the hot center of the wafer to expand more quickly than the cooler edges. As a result, the wafer warps.
If the wafers are not flat on the pedestal when processing starts, a layer of material can be deposited on the under side of the wafer, which would touch the pedestal if the wafer were not warped. Also, the warping of wafers may affect the uniformity of the layer deposited. Backside deposition and nonuniform deposition are unacceptable because they may harm the operating characteristics of the device. Thus, the wafers must be allowed to sit on the heated pedestal long enough for the wafers to relax before processing can begin. It may take 80 to 120 seconds for the wafers to relax. Thus, allowing the wafer to relax on the pedestal takes time that decreases processing throughput. One alternative method of preventing backside deposition and nonuniform deposition is to prevent wafer bowing by clamping the wafer to the pedestal during the heating. As the clamped wafer heats up and expands, the contact force between the wafer and the pedestal created by the clamp causes small scratches on the backside of the wafer and causes the backside of the wafer to pick up particles from the pedestal. Such scratches and particles on the back side of the wafer are undesirable as they can affect the thickness of the wafer, which can cause depth-of-focus inaccuracies in subsequent photolithography steps.
A method is provided for reducing the amount of bowing in a wafer when the wafer is heated to processing temperature and for reducing the amount of time required to heat the wafer to processing temperature. In one embodiment, before the wafer is loaded on the heated pedestal for processing, a heat transfer gas with a thermal conductivity and mean free path greater than that of oxygen, such as helium, is introduced into the processing chamber. The wafer is then loaded and heated in heat transfer gas. In another embodiment, the wafer is loaded on the heated pedestal, then the contents of the processing chamber are evacuated until the chamber is at a pressure of less than 0.1 Torr. The wafer is then heated to process temperature in the low pressure environment.